1. Field of the Invention
The present invention relates to a semiconductor integrated circuit device composed of an MOSFET for use in a circuit that operates through DC drive or low-frequency drive.
2. Description of the Related Art
FIG. 2 is a schematic sectional view showing a conventional semiconductor device. A gate electrode 13 formed of N+ type polycrystalline silicon is formed above a P-well layer 2 on an N-type semiconductor substrate 1 through a gate insulating film 8 to thereby form an N-channel MOS transistor (hereinafter, referred to as NMOS) 102. Another gate electrode 13 similarly formed of N+ polycrystalline silicon is formed above the N-type semiconductor substrate 1 through the gate insulating film 8 to thereby form a p-channel MOS transistor (hereinafter, referred to as PMOS) 103. In other words, both of the transistors constitute a complementary MOS structure (hereinafter, referred to as CMOS). In regions of the P-well layer 2 outside both sides of the gate electrode 13 in the NMOS 102, source/drain regions 5 are formed. In regions of the N-type semiconductor substrate 1 outside both sides of the gate electrode 13 in the PMOS 103, source/drain regions 7 are formed. A field insulation film 3 is formed for element isolation between the NMOS 102 and the PMOS 103.
In general, those MOSFETs have been used in the circuit operating through the DC drive or low-frequency drive.
The MOS transistor generally adopts a silicon surface right below the gate insulating film as a channel and thus, tends to suffer influences of a defect existing at an interface between the gate insulating film and the silicon or a surface level. When operating the transistor, in particular, through the DC drive or low-frequency drive at 500 kHz or lower, exchanges of the electrons and holes frequently occur because of the above surface level and defect, which appears in the form of noise against a high frequency signal, i.e., so-called 1/f noise. This causes a serious problem in a semiconductor integrated circuit. The defect or surface level develops due to dangling bonds resulting from discontinuous atomic bonds between silicon dioxide and silicon, for example, constituting the gate insulating film; a defect exiting on the silicon surface prior to the formation of the gate insulating film; a deposition thereon; and the like. The complete removal thereof is hardly attained.
The noise generated due to the defect or the surface level increases as the gate of the MOS transistor reduces its area. That is, provided that a channel length of the MOS transistor is represented by L and a channel width thereof is represented by W, the noise becomes more conspicuous during the operation at a high frequency with a decrease in value of L×W.
Among various MOS transistors, a surface channel MOS transistor exhibits such a tendency more remarkably than a buried channel MOS transistor does. This is because the channel of the surface channel transistor is formed at the interface between the gate insulating film and the silicon, but the channel of the buried channel transistor is formed deep inside the silicon as viewed from the interface between the gate insulating film and the silicon, so that the buried channel transistor hardly suffers the aforementioned influences of the defect and the surface level. Namely, in the MOS transistor having the gate electrode formed of the N+ polycrystalline silicon, the NMOS serving as the surface channel transistor involves more noise components against signal components than the PMOS serving as the buried channel transistor.
On the other hand, in general, the PMOS is inferior to the NMOS in driving power. This results from the fact that carriers constituting a drain current are electrons in the NMOS but are holes in the PMOS. That is, a mobility of the hole corresponds to about ⅓ of that of the electron and hence, the driving power of the PMOS results in about ⅓ of that of the NMOS. Therefore, generally in the semiconductor integrated circuit for which the high driving power is required with a small area, the NMOS is used in many cases rather than the PMOS.
However, as described above for the prior art, the NNOS generally serves as the surface channel transistor and therefore suffers such a problem as an increased noise.
Alternatively, for the purpose of relieving the influence of the noise, the application of a method of increasing the gate area of the NMOS or a method of using the PMOS in many cases causes an increase in element area, which may lead to an increased chip area. As a result, there arises a problem in that the cost is increased.